This invention relates to a method for the production of cellulose and particularly to a method for the production of cellulose which can be used in paper-making and in textiles. More specifically, this invention relates to a pre-treatment of wood chips prior to a digestion process whereby the digestion process can be carried out economically, the cellulose fibers do not require extensive bleaching with bleaching agents and the cellulose polymeric chain is reduced in size whereby the digestion process can be carried out more efficiently. This invention is also directed to a process of pre-treatment of such cellulose containing chips whereby the following digestion process can be accelerated and the resultant cellulose is obtained in improved yield and has improved strength properties.
It is known in the preparation of cellulose fibers for use in paper-making to subject a naturally occuring material containing cellulose, e.g., wood chips to a full chemical or semi-chemical process for the purpose of separating the cellulose fibers therein from other naturally occuring components. In such chemical process, the wood chips are cooked with suitable chemicals in an aqueous solution, usually at elevated temperatures and pressures. The object is to dissolve the naturally occuring lignin and other extraneous compounds, leaving the cellulose intact and in fibrous form. The objective can be realized to a commercially satisfactory degree.
It is known to utilize, in such a digestion process, sulfites or sulfates. Both methods suffer from the disadvantage that the lignin content in the unbleached cellulose, as a function of the required processing properties, cannot be decreased below a certain level. Because of this, the unbleached cellulose has a low degree of whiteness. In the production of bleached cellulose, the relatively high use of bleaching chemicals to improve the whiteness properties of the unbleached cellulose in turn causes damage to the cellulose fiber itself and impairs the cellulose's ability to withstand certain processing.
It is known according to West German Offenlegenschrift No. 2,208,335 to pre-treat the wood chips by irradiation with charged particles. The purpose of this pre-treatment is to improve the yield and strength of the cellulose for use in papermaking. When the chips are irradiated with high energy particles, however, structural alterations occur at the macro-molecules of the lignin. This structural alteration makes elution of this substance far more difficult owing to the fact that a lignin condensation during the digestion process occurs. This lignin condensation effects a higher lignin content in the unbleached cellulose. This disadvantage in increased lignin content in the unbleached cellulose in turn requires the use of still higher amounts of bleach chemicals during the bleaching process. The use of such higher amounts of chemicals in turn lowers the strength of the cellulose. The effect of increased cellulose strength which is the purpose behind the irradiation is thereby eliminated at least in part by the subsequent bleaching operation. When unbleached cellulose produced by this method is used for paper-making, the whiteness of the end product is reduced.
It is also known according to East German WP No. 78,689, issued December 20, 1970, that the pre-treatment of wood chips with aqueous bi-sulfite or sulfite solutions of an alkali metal, alkaline earth metal or ammonium or mixtures of such solutions can be performed to reduce the disadvantageous alterations of lignin during the storage of the chips prior to digestion. By this method, the alterations of the lignin during exposed storage of the chips can be prevented. However, while the lignin does not undergo the condensation referred to above, the lignin content in the unbleached cellulose still cannot be lowered below those levels typically encountered in the known sulfite digestion processes.
It has, therefore, become desirable to provide a process for the pre-treatment of cellulosic containing chips or other materials whereby the lignin in the unbleached cellulose following digestion is reduced. More specifically, it has become desirable to provide a process for the pretreatment of such naturally occuring materials, prior to a digestion process, whereby during the digestion process, the lignin does not undergo a lignin condensation. Stated in the positive, it has become desirable to provide a pretreatment process whereby the finally-produced unbleached cellulose has improved strength properties and improved whiteness. It has also become desirable to provide such a process which enables the bleaching process to be carried out with far less chemicals whereby to preserve the processing characteristics of the cellulose fibers.